1. Field of the Invention
The present invention relates to syringes and more particularly concerns disposable syringes having single-use features.
2. Description of Related Information
In the United States and throughout the world the multiple use of hypodermic syringe products which are intended for single use only is instrumental in drug abuse and more particularly in the transfer of contagious diseases. Intravenous drug users who routinely share and reuse syringes are a high risk group with respect to the AIDS virus. Also, the effects of multiple use are a major concern in third world countries where repeated use of syringe products may be responsible for the spread of many diseases. Reuse of single use hypodermic syringe assemblies is also instrumental in the spread of drug abuse even in the absence of infection or disease.
Many attempts have been made to remedy this problem. Most notable are early contributions which relied on a specific act to destroy the syringe after use either by using a destructive device or providing a syringe assembly with frangible zones so that the syringe could be rendered inoperable by the application of force. Other attempts involve the inclusion of structure which would allow the destruction or defeating of the syringe function through a conscious act by the syringe user. Although many of these devices work quite well they do require the specific intent of the user followed by the actual act to destroy or render the syringe inoperable. None of these devices is effective with a user having the specific intent to reuse the hypodermic syringe. Accordingly, there is a need for a single use hypodermic syringe which becomes inoperative or incapable of further use automatically without any additional act on the part of the user. This automatic function is much harder to provide because the means for rendering the syringe inoperable must not prevent its filling or use under normal conditions.
Along these lines, U.S. Pat. No. 4,367,738 to Legendre et al. teaches a syringe having a barrel with a restricted open end which interacts with a plunger rod having a plurality of short flexible spikes attached at an oblique angle away from the piston. During injection, the oblique spikes will pass through the restricted opening in the barrel but the plunger rod motion in a reverse direction will be prevented by the oblique spikes. The syringe of Legendre et al. is suitable only for prefilled use because the plunger rod can not be fully withdrawn. The plunger rod of this syringe may be moved back and forth, as to reuse, for the distance between tips on the spike. For example, if the tips on adjacent spikes are one centimeter apart, when measured along the axis of the plunger rod the piston would appear to be cyclable back and forth for a distance slightly less than one centimeter. This syringe of Legendre et al. must have many spikes along the axis of the plunger rod to be rendered unreusable as a practical matter. A solution to the problem of multiple use which only addresses prefilled syringes is limited in nature because millions of injections each year are performed with vaccines that must be used a short time after being placed into injectable form by mixing with water.
U.S. Pat. No. 4,781,683 to Wozniak et al. teaches a single-use syringe that is rendered inoperative without requiring any deliberate action on the part of the user. Wozniak et al. accomplished this result by providing a hydrophilic expansion plug which is positioned in the outlet channel of the syringe which expands a short time after being exposed to a drug containing water to render the syringe inoperative. The syringe of Wozniak et al. may not function in a prefilled mode wherein the syringe is filled through the hypodermic needle because the syringe would probably be rendered inoperative before the time of use. Also, in many hospital applications, syringes are prefilled through the delivery tip in the hospital pharmacy and sent to the nursing stations for use immediately or during the next several hours or days. It would appear that the syringe of Wozniak et al. would not be applicable to this use because it would be rendered inoperative before time of use unless the hydrophilic expansion plug required an extended amount of time to reacts, and in this latter case it would not be a single use syringe.
Another single-use hypodermic syringe is taught by Trenner in U.S. Pat. No. 4,781,684. Trenner appears to have overcome many of the shortcomings of the prior art with respect to the most common application of hypodermic syringes which is syringes being filled through the needle tip at the time of use or within a short period of time before use. The syringe of Trenner appears to allow for the easy filling and substantially unrestricted use. Trenner provides an annular groove near the distal end of the syringe barrel having a diameter which is greater than the diameter of the cylindrical inner surface of the barrel. A locking element, positioned between the stopper and the plunger rod, has an outside diameter which is larger than the diameter of the barrel so that when the syringe stopper is moved to its furthest most position inside the barrel, expelling medicament from the syringe, the locking element engages the annular groove and prevents withdrawal of the stopper. The design appears to allow the forced disconnection of the plunger rod from the stopper while still retaining the stopper in the barrel. While the approach of Trenner overcomes many of the shortcomings of the prior art the Trenner syringe can be recycled many times so long as the user is careful not to move the stopper all the way to the distal end so as to engage the locking element with the annular groove.